I am attempting to use a RTL2832U based DVB-T dongle as a "fast" ADC for recording baseband signals. gr-osmosdr under GNURadio 3.7 (as provided by Debian GNU/Linux) provides the expected samples when using an E4000 (zero IF) dongle. Being obsolete, such a dongle is no longer relevant, and I want to do the same with the (non-zero IF) R820T(2) dongles. From http://sdr.osmocom.org/trac/wiki/GrOsmoSDR I should be able to use the direct_samp=1|2 option, but that does not work. I kept pins 1 and 2 connected to the R820T output and connected a wire to pin 5 (Q-) while Q+ (pin 4) is short circuited to pin 3 (gnd). A signal generator outputs a periodic signal (150 mV amplitude, 100 kHz) but still no baseband signal, and on the FFT of the osmosdr output I still see the spectrum of the FM band when tuning to 100 MHz rather than the samples from the (initially unconnected) Q input. Any idea if the "Device Arguments" of osmosdr-source is actually parsed and interpreted ? or is there a command line way of acquiring the raw baseband I/Q data ?
Thanks, JM
The Q pins are unused, the r820t(2) is a low IF tuner. Try connecting your antenna to I (Pin 1&2) instead.
-H
Hi Jean-Michel,
On 28.10.2016 08:07, Jean-Michel FRIEDT wrote:
(150 mV amplitude, 100 kHz) but still no baseband signal, and on the FFT of the osmosdr output I still see the spectrum of the FM band when tuning to 100 MHz rather than the samples from the (initially unconnected) Q input.
The problem is that by enabling the direct sampling mode, the RTL2832 is forced to low-IF mode and only one ADC is enabled.
For your purpose, the device needs to be in zero-IF mode, which normally wouldn't be used if a low-IF tuner like the R820T is attached.
Please see the attached patch, if you call rtlsdr_set_direct_sampling with 1 after applying it should work.
Regards, Steve
Quick update thanks to the long week end in France which left enough time to test all combinations of hardware and software: 1/ I tried applying the patch provided by Steve on this list on librtlsdr and compiling gr-osmosdr for running in gnuradio-companion. I know I am running the right source block linked with the right library (added debug messages) => no signal on a R820T2 dongle connected to a signal generator (I+), the other pin (I-) either grounded or decoupled with a 100 nF cap. 2/ doing the same on a E4k based dongle also provided only noise and no signal, a strange result since in the very beginning of my experiments (2012) I had used this same dongle for direct sampling. Being a zero-IF, I could just bypass the E4k mixer and record the raw samples fed to I+ and Q+ (I- and Q- being grounded). Surprisingly, running the same gnuradio-companion flowgraph (osmosdr source -> Qt time sink) with NO direct sampling option does yield correct samples, so the hardware is sound and functional. 3/ E4k no longer being available, I have to find a way to compensate for the R820T IF: I checked using no option in osmosdr source that I could get some relevant signal when tuning the signal generator around the IF frequencies (4.8 MHz ? 5 MHz ? difficult to say with all the aliases), while the option "rtl=0, direct_samp=1" always returns noise.
I am not familiar enough with the RTL2832U configuration registers to mess with them myself, but I believe direct sampling in not functional at the moment in librtlsdr. I might be wrong of course.
Thanks, JM
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ew -
Jean-Michel FRIEDT -
jmfriedt -
Steve Markgraf